1. Field of the Invention
The present invention is generally directed to broadband communication systems. In particular, the present invention is directed to a method and apparatus for transmitting information over the physical layer of a broadband communication system, such as a cable modem system.
2. Background
In conventional cable modem systems, a coaxial or hybrid fiber-coaxial (HFC) network provides a point-to-multipoint topology for supporting bidirectional data communication between a cable modem termination system (CMTS) at the cable headend and multiple cable modems at the customer premises. The communication path from the CMTS to the cable modems is typically referred to as the downstream, while the communication path from the cable modems to the CMTS is typically referred to as the upstream.
DOCSIS (Data Over Cable Service Interface Specification) refers to a group of specifications published by CableLabs that define industry standards for cable headend and cable modem equipment. In part, DOCSIS sets forth requirements and objectives for various aspects of cable modem systems including operations support systems, management, data interfaces, as well as network layer, data link layer, and physical layer transport for data over cable systems. The current version of the DOCSIS specification is version 2.0, and includes the DOCSIS Radio Frequency Interface (RFI) Specification SP-RFIv2.0-I03-021218 (hereinafter, “the DOCSIS RFI Specification”), the entirety of which is incorporated by reference herein.
One difference between DOCSIS 2.0 and earlier versions of the standard is that two different burst type formats are supported for upstream physical (PHY) layer transmissions between cable modems and the CMTS. In particular, as specified by the DOCSIS RFI Specification, the upstream physical media dependent (PMD) sublayer can use either an FDMA/TDMA burst type format, also referred to as the “TDMA mode,” or an FDMA/TDMA/S-CDMA burst type format, also referred to as the “S-CDMA mode.” Whether the TDMA mode or S-CDMA mode is used by a cable modem is configured by the CMTS through the transmission of DOCSIS MAC (media access control) messages.
FDMA (frequency division multiple access) indicates that multiple RF channels are assigned in the upstream band. A cable modem transmits on a single RF channel unless reconfigured to change channels. TDMA (time division multiple access) indicates that upstream transmissions have a burst nature. A given RF channel is shared by multiple cable modems via the dynamic assignment of time slots. S-CDMA (synchronous code division multiple access) indicates that multiple cable modems can transmit simultaneously on the same RF channel and during the same TDMA time slot, while being separated by different orthogonal codes.
In accordance with the DOCSIS RFI Specification, the upstream modulator of a DOCSIS cable modem can utilize Trellis Coded Modulation (TCM) for modulating upstream bursts in S-CDMA mode only. As will be appreciated by persons skilled in the relevant art(s), TCM refers to a modulation technique that incorporates redundancy into a modulation constellation to attain coding gain. In TCM, a symbol transition rule is used that maximizes the Euclidean distance between successive transmissions in the modulation signal space.
According to the DOCSIS RFI Specification, TCM cannot be used for upstream communication in the TDMA mode. However, it has been observed that the use of TCM in the TDMA mode would also provide significant benefits in terms of coding gain, particularly when combined with forward error correction (FEC) techniques such as Reed Solomon error correction and byte interleaving.
What is desired then, is a cable modem method and apparatus that permits TCM to be used for modulating upstream bursts while operating in the TDMA mode of a DOCSIS 2.0 system. The desired method and apparatus should permit a cable modem to optionally enable or disable TCM while operating in the TDMA mode, thereby permitting the cable modem to inter-operate with enhanced headend equipment that is designed to process TCM modulated signals in the TDMA mode as well as with conventional DOCSIS-compliant headend equipment that is not designed to process such signals.